Optical communications systems (e.g., integrated optic or multi-functional chips) now routinely employ electro-optic devices that utilize electrodes to modulate optical signals propagating through a waveguide formed in an optically transmissive substrate and optically coupled between an input optical fiber and one or more output optical fibers. The substrate typically comprises an electro-optic crystal, such as lithium niobate (LiNbO3), that transmits substantially polarized light and undergoes a change in refractive index when exposed to an electrical voltage. Generally in such optical modulators, one or more waveguides may be formed proximate the upper surface of the substrate, and one or more surface electrodes are deposited on the surface proximate the waveguides. When a voltage is applied to the substrate via the surface electrodes, light propagating through the substrate is modulated thus producing a phase modulated optical signal.
The quality of a waveguide device's emitted optical signal is strongly influenced by the ability of the device to confine propagating light to a single polarization mode (e.g., transverse magnetic or transverse electric mode), a characteristic which is commonly referred to as the polarization extinction ratio (PER). In one known waveguide device, specifically a proton exchange polarizer, the PER is increased employing a spatial filter to block two optical paths (i.e., the primary and secondary optical paths) that the unguided transverse magnetic (TM) mode light may travel and be received at the output fiber, thus interfering with the desired optical signal. The spatial filter may utilize barriers (e.g., baffles or grooves cut into the substrate by, for example, a dicing saw) positioned at primary and secondary reflections points at the bottom of the substrate to substantially block their optical paths. Alternatively, the barriers may be regions comprising a material having a different index of refraction such that TM mode light passing therethrough is directed away from the waveguide. In the above referred to waveguide device, three barriers are disposed along the bottom surface of the substrate at locations equidistance from one another and the terminal ends of the substrate; i.e., for a substrate having a length of X, the first filter is disposed a distance (¼)X from a first end of the substrate, the second filter is disposed a distance (¼)X from the first filter and (½)X from the first end, and the third filter is disposed a distance (¼)X from the second filter and ¾X from the first end.
Unfortunately, spatial filter arrangements of the type described above typically block only two optical paths (i.e., the primary and secondary paths) traveled by unguided light. Thus, unguided light may still reach the output of the waveguide by a number of alternative paths. If the waveguide is bifurcated (i.e., a single input fiber feeds two output fibers), these alternative paths include the two paths traveling directly from the input fiber to each of the output fibers. Furthermore, known spatial filter arrangements do not maximize the number of obstructed light paths relative to the number of employed barriers; that is, the three barriers utilized in the known device block out two unguided light modes, the primary and secondary mode. The filter is redundant in that the first and third barriers are located at two different reflection points of the secondary optical path. As a result, known spatial filter arrangements are inefficient and do not provide optimal price-to-performance ratios.
It should thus be appreciated that it would be desirable to provide a waveguide device having a spatial filter arrangement capable of blocking more than two unguided optical paths. It should also be appreciated that it would be desirable that the waveguide be configured to maximize the number of unguided light paths that are blocked relative to the number of barriers employed. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.